In this type of apparatus, the blast chamber includes means such as a cylinder and a piston for compressing the gas during an opening or current-interrupting maneuver, and for directing the compressed gas through a blast nozzle onto the arc to be extinguished.
The term "thermal volume" is used to designate a volume opening out to a zone close to where an arc is struck when the circuit breaker opens.
The gas in this volume is heated by the arc, and as a result its pressure increases.
The energy accumulated in this way is generally used for contributing to the arc-blasting effect and/or for constituting additional energy for maneuvering the circuit breaker.
When the current to be interrupted is a low current (i.e. the nominal current or less), the arc is small and the thermal volume has a negligible effect on interrupting the arc, so the entire arc-interrupting effect is produced by the blast chamber.
In contrast, if the current to be interrupted is high (a short circuit-current) the arc is large and a considerable amount of energy is accumulated in the thermal volume.
In some prior art circuit breakers, the gas heated by the arc is applied directly to the arc that needs to be extinguished. Unfortunately, such recently-heated hot gas does not have the necessary dielectric qualities for being a good arc-extinguishing gas, and in particular it contains various impurities and some of its molecules are ionized.
An aim of the invention is to use the energy of the arc to direct a jet of cold gas onto the arc rather than a jet of polluted hot gas.
Use has sometimes been made of the arc-heated gas to thrust cold gas onto the arc by means of a piston. One aim of the invention is to thrust said cold gas without using mechanical means.